1. Field of the Invention
This invention relates to a technique for driving a liquid crystal display device, and more particularly to a liquid crystal panel driving method of driving a liquid crystal panel using an inversion system and an apparatus thereof.
2. Description of the Prior Art
Generally, a liquid crystal display device controls the light transmissivity of liquid crystal cells in a liquid crystal panel to display a picture corresponding to a video signal. Such a liquid crystal display device uses a line-inversion system, a column-inversion system, a dot-inversion system and a group-inversion system, etc. so as to drive the liquid crystal cells in the liquid crystal panel. In a liquid crystal panel driving method of line-inversion system, as shown in FIG. 1A and FIG. 1B, the polarities of data signals applied to the liquid crystal panel are inverted in accordance with row lines, that is, gate lines on the liquid crystal panel and in accordance with frames. In a liquid crystal panel driving method of column-inversion system, as shown in FIG. 2A and FIG. 2B, the polarities of data signals applied to the liquid crystal panel are inverted in accordance with column lines, that is, source lines on the liquid crystal panel and in accordance with frames. In a liquid crystal panel driving method of dot-inversion system, as shown in FIG. 3A and FIG. 3B, data signals having polarities contrary to the adjacent liquid crystal cells on the gate lines and to the adjacent liquid crystal cells on the data lines are applied to each liquid crystal cells in the liquid crystal panel, and the polarities of data signals applied to all liquid crystal cells in the liquid crystal panel are inverted every frame. In other words, in the dot-inversion system, data signals are applied to the liquid crystal cells in the liquid crystal panel in such a manner that the positive(+) polarity and the negative(−) polarity appear alternately as shown in FIG. 3A as it goes from the liquid crystal cell at the left upper end into the liquid crystal cells at the right side and into the liquid crystal cells at the lower side when a video signal in the odd-numbered frame is displayed; while data signals are applied to the liquid crystal cells in the liquid crystal panel in such a manner that the positive (+) polarity and the negative(−) polarity appear alternately as shown in FIG. 3B as it goes from the liquid crystal cell at the left upper end into the liquid crystal cells at the right side and into the liquid crystal cells at the lower side when a video signal in the even-numbered frame is displayed.
The line-inversion system in the above-mentioned liquid crystal panel driving method has a serious crosstalk in the horizontal direction. Particularly, when a picture alternated with two colors (i.e., a color with a medium gray scale and a black color) depending on the line is displayed on the liquid crystal panel by the liquid crystal panel driving method of line inversion system, a serious flicker emerges between the horizontal lines. Similarly, when a picture alternated with two colors (i.e., a color with a medium gray scale and a black color) depending on the line is displayed on the liquid crystal panel by the liquid crystal panel driving method of column inversion system, a serious crosstalk in the vertical direction is generated. The dot-inversion system in which the polarities of the data signals are inverted in both the vertical and horizontal directions unlike the line-inversion system and the column inversion system provides better picture quality than the line- and column-inversion systems. Recently, owing to such an advantage, the liquid crystal panel driving method of dot-inversion system has been often used.
However, the liquid crystal panel driving method of dot-inversion system has a problem in that a brightness difference is generated at a boundary portion between column driver integrated circuits (IC'S). This generation of the brightness difference at the boundary portion between the column driver IC's is caused by an output deviation of the column driver IC's and a large difference in a voltage Vgs between the gate and the source of a thin film transistor (TFT) generated because the polarities of video signals applied to the liquid crystal cells at the boundary portion between the column driver IC's is opposed to each other.
The foregoing will be described in detail with reference to FIG. 4 to FIG. 6B. Each pixel on the liquid crystal panel can be expressed by an equivalent circuit as shown in FIG. 4. In FIG. 4, the pixel includes a TFT connected between a gate line GL and a data line DL, and a liquid crystal cell Clc connected between a source terminal of the TFT and a common voltage line CL. Further, the pixel includes a parasitic capacitor Cgs formed between the source terminal of the TFT and the gate line GL, and a parasitic resistor Rtft existing between the drain terminal and the source terminal of the TFT. The parasitic resistor Rtft is an equivalent resistance between the drain terminal and the source terminal when the TFT is turned off, which does not have a fixed value. The liquid crystal cell Clc charges a difference voltage between a video signal at the data line DL and a common voltage Vcom applied to the common voltage line CL until a time interval when the TFT maintains an ON state, that is, until a time interval when a gate high voltage Vgh is applied to the gate line GL. Accordingly, the difference voltage charged in the liquid crystal cell Clc becomes different depending on the polarity of the video signal and an output deviation of the column driver IC's.
Referring to FIG. 5A and FIG. 5B, there are shown a liquid crystal panel 10 having liquid crystal cells arranged in a matrix type, N column driver IC's 12 for individually applying a video signal to M data lines DL, and J gate driver IC's 14 for individually driving K gate lines GL. Herein, J, K, M and N are an integer. The N column driver IC's 12 apply video signals with a contrary polarity to the adjacent data lines DL in such a manner to be synchronized with a time interval when the gate high voltage Vgh is sequentially applied to the gate lines GL by the J gate driver IC's 14. The pixels at the oblique-lined boundary portion positioned between the column driver IC's 12 are supplied with video signals having a polarity contrary to the video signals applied to the adjacent pixels. Voltages charged in the odd-numbered pixels P(J*(2I−1),M), P(J*(2I−1),2M), . . . , P(J*(2I−1), (N−1)*M) and P(J*(2I−1),M+1), P(J*(2I−1),2*M+1), . . . , P(J*(2I−1), (N−1)*M+1) in the data line direction of the pixels P(J*K,M), P(J*K,2M), . . . , P(J*K,(N−1)*M) connected to the left data lines DL and the pixels P(J*K,M+1), P(J*K,2*M+1), . . . , P(J*K,(N−1)*M+1) connected to the right data line DL at the boundary portions of the adjacent column driver IC's 12 in the odd-numbered frames as shown in FIG. 5A are represented by FIG. 6A and FIG. 6B, respectively. Herein, I is an integer. FIG. 6A represents a voltage Vgs between the gate and the source charged in the odd-numbered pixels P(J*(2I−1),M), P(J*(2I−1),2M), . . . , P(J*(2I−1), (N−1)*M) connected to the left data lines at the boundary portions between the column driver IC's 12, whereas FIG. 6B represents a voltage Vgs between the gate and the source charged in the odd-numbered pixels P(J*(2I−1),M+1), P(J*(2I−1),2*M+1), . . . , P(J*(2I−1), (N−1)*M+1) connected to the right data lines at the boundary portions between the column driver IC's 12 in the odd-numbered frames. As seen from FIG. 6A and FIG. 6B, the voltage Vgs between the gate and the source charged in the odd-numbered pixels P(J*(2I−1),M), P(J*(2I−1), 2M), . . . , P(J*(2I−1), (N−1)*M) connected to the left data lines at the boundary portions in the odd-numbered frames becomes much smaller than the voltage Vgs between the gate and the source charged in the odd-numbered pixels P(J*(2I−1),M+1), P(J*(2I−1),2*M+1), . . . , P(J*(2I−1), (N−1)*M+1) connected to the right data lines at the boundary portions. A difference in the voltages Vgs between the gates and the sources charged in the adjacent pixels within the boundary portions emerges at the even-numbered pixels P(J*2I,M), P(J*2I,2M), . . . , P(J*2I, (N−1)*M) and P(J*2I,M+1), P(J*2I,2M+1), . . . , P(J*2I,(N−1)*M+1) in the data line direction in opposition to the odd-numbered pixels P(J*(2I−1),M), P(J*(2I−1),2M), . . . , P(J*(2I−1), (N−1)*M) and P(J*(2I−1),M+1), P(J*(2I−1),2*M+1), . . . , P(J*(2I−1), (N−1)*M+1), respectively. Accordingly, since a brightness difference of the displayed picture becomes serious at the boundary portions between the column driver IC's 12, a noise pattern at the vertical line emerges on the field. Such a phenomenon becomes more serious as the output deviation of the column driver IC's 12 goes larger. The video signal applied to each pixel cell in the even-numbered frames following the odd-numbered frames has a polarity contrary to that in the odd-numbered frames. In the even-numbered frames, a voltage Vgs between the gate and the source charged in the odd-numbered pixels P(J*(2I−1),M), P(J*(2I−1),2M), . . . , P(J*(2I−1), (N−1)*M) connected to the left data lines at the boundary portions between the column driver IC's 12 is as shown in FIG. 6B. On the other hand, a voltage Vgs between the gate and the source charged in the odd-numbered pixels P(J*(2I−1),M+1), P(J*(2I−1),2*M+1), . . . , P(J*(2I−1), (N−1)*M+1) connected to the right data lines at the boundary portions between the column driver IC's 12 is as shown in FIG. 6A. Accordingly, in the even-numbered frames, the voltage Vgs between the gate and the source charged in the odd-numbered pixels P(J*(2I−1),M), P(J*(2I−1),2M), . . . , P(J*(2I−1), (N−1)*M) connected to the left data lines at the boundary portions becomes much smaller than the voltage Vgs between the gate and the source charged in the odd-numbered pixels P(J*(2I−1),M+1), P(J*(2I−1),2*M+1), . . . , P(J*(2I−1), (N−1)*M+1) connected to the right data lines at the boundary portions. In the even-numbered frames, a difference in the voltages charged in the even-numbered pixels P(J*2I,M), P(J*2I,2M), . . . , P(J*2I, (N−1)*M) and P(J*2I,M+1), P(J*2I,2M+1), . . . , P(J*2I,(N−1)*M+1) in the data line direction emerges in opposition to that in the odd-numbered pixels P(J*(2I−1),M), P(J*(2I−1),2M), . . . , P(J*(2I−1), (N−1)*M) and P(J*(2I−1),M+1), P(J*(2I−1),2*M+1), . . . , P(J*(2I−1),(N−1)*M+1), respectively.
Meanwhile, a large output deviation may be generated between the output terminals of the same column driver IC 12. In this case, a large brightness difference is generated between the adjacent pixels in the data line direction of the pixel areas within the same column driver IC 12 in similarity to the above-mentioned phenomenon appearing at the boundary portions between the column driver IC's 12.
As a result, in the conventional dot-inversion system, a voltage difference and a current difference charged in the adjacent pixels in the data line direction becomes large, and a large brightness difference is generated between the adjacent pixels in the data line direction due to an output deviation within the column driver IC 12 or an output deviation between the column driver IC's 12 which is more increased at a higher resolution.